1. Field of the Invention
The present invention is in the field of brushless alternating current synchronous electrical machines in which heteropolar field excitation is supplied from field windings or permanent magnets contained in the stator.
2. Description of the Related Art
In an MS thesis published in 1958 and in two technical papers published in October, 1960, A. K. Das Gupta described a then new implementation of a three-phase heteropolar inductor electrical machine. These references are:
1. A. K. Das Gupta "An Analysis of One type of Inductor Alternator", MS Thesis, University of Wisconsin, Madison, Wis., 1958. PA1 2. A. K. Das Gupta, "An Analytical Method to Find the Best Number of Stator and Rotor Teeth of Inductor Alternator for 3-Phase Sinusoidal Voltage Generation", AIEE Trans. on Power Apparatus and Systems, Vol. 79, Oct. 1960, pp. 674-679. PA1 3. A. K. Das Gupta, "Mathematical Analysis of Inductor Alternators", AIEE Trans. on Power Apparatus and Systems, Vol. 79, Oct. 1960, pp. 684-689.
Gupta and a colleague, P. K. Dash, later published three additional papers on the design and analysis of this type of machine in the November 1969 IEEE Transaction on Power Apparatus and Systems, but the essence of the machine operation is fully disclosed in Gupta's first three publications. As originally described by Gupta, the typical three-phase machine structure is shown in FIG. 1. An inner rotor of laminated electrical steel construction with seven evenly spaced teeth on its outer surface is enclosed by a laminated stator, which has twelve evenly spaced teeth and slots facing the rotor. The stator slots contain the machine windings. As shown in FIG. 1, these windings form coils, each coil subtending two stator teeth. The coils, twelve in all, are interleaved with one another. Six of the coils, P1 through P6, are stator phase coils and conduct AC phase currents, two coils per phase. The remaining six coils, F1 through F6, alternating in position with the phase coils, are field coils which conduct direct current (DC) and can all be separately driven, or jointly driven, connected in series or in shunt, or in some series/shunt combination. The current in these field coils excites field magnetic flux, which then flows in closed paths in the stator and rotor structures. The value and sign of this field flux in any particular path in the stator is dependent on the position of nearby rotor teeth. For example, in the linear or developed machine cross-section of FIG. 2, current in field coil F1 in the direction shown will produce field flux A flowing in the path across the air gap from stator tooth S1 to rotor tooth R1. If the rotor advances one stator tooth pitch to the position shown in FIG. 3, then flux A flowing in rotor tooth R1 reverses and switches its stator path from stator tooth S1 to stator tooth S2. The direction of flux A through stator phase coil P1 reverses since the source of its field excitation also switches from the ampere turns in field coil F1 to the ampere turns in field coil F2. Thus for continuous rotor motion, as rotor teeth pass stator teeth, alternating polarity field flux is linked by stator phase coils due to stator field winding DC ampere turns. The variation of field flux linked by the different stator phase coils is in exact time and position synchronism with the variation of rotor movement across the face of the stator. Therefore this machine structure is termed an AC synchronous motor or generator (alternator) dependent on the method of energy conversion, electrical to mechanical or mechanical to electrical.
A complete electrical cycle of operation for the machine FIG. 1 takes place over rotor physical movement of one rotor tooth pitch. Since there are seven rotor teeth there will be seven electrical cycles of operation for each complete rotation of the rotor. This machine is then said to be a fourteen (14) magnetic pole device, with two magnetic poles associated with each AC electrical cycle of operation.
It is desirable in any AC electrical machine to operate in a manner such that the torque acting on the machine elements, created by interaction between the field fluxes and the stator phase winding currents, be a maximum for any given set of values of the phase currents, in either motor or generator mode of operation. In addition, for mechanical and thermal considerations, machine construction should be that which promotes equal levels of energy conversion in any given phase averaged over an electrical cycle for that phase. That is, the machine should be electrically balanced among its phases. Considering any one phase alone, it is also desirable that balance be achieved in the energy conversion process as measured over any given half electrical cycle within that phase. Thus, it is desirable in AC machine design to achieve equal and balanced peak torque per amp performance within a half cycle of operation of any given phase.
The machine construction of FIG. 1 does not lend itself to peak attainable and balanced operation within half cycles of any given phase. As shall be shown, the reason for this is non-electrical symmetry, as seen in any one phase, due to the requirement of equal spacing between teeth on both the stator and rotor structures.
For ease of explanation, let the angular separation between teeth be measured in units that will be integral in value for both the stator and rotor. The largest angular value for this measuring unit, denoted as m, is then 360/(7.times.12) degrees. The tooth separation or pitch of the rotor is 12 m and the pitch of the stator teeth is 7m. One rotor tooth pitch is equivalent to 360 electrical degrees so in terms of electrical degrees m=360/12=30.degree.(e), with the notation .degree.(e) signifying degrees in an electrical cycle.
The structure of FIG. 1 is not electrically balanced over any one half cycle because the stator teeth are separated by 7m or 210.degree.(e). Thus, rotor movement of 6m or 180.degree.(e) (i.e. movement equivalent to one half electrical cycle) would not completely reverse the magnetic field flux linking any given stator phase. It takes a 7m move of the rotor to completely reverse the flux within any given phase coil. This 7m move is shown in the beginning and end of movement diagrams of FIGS. 2 and 3, respectively. The voltage induced in any one phase coil due to the field flux change over a complete electrical cycle will have a fundamental component at the fundamental frequency, the inverse of the time extent of an electrical cycle, but it will also have higher harmonic terms due to the non balance between the two half cycles of operation. The harmonic terms will not contribute to the energy conversion process unless they can be matched with corresponding phase current harmonics, not an easily achievable process, and thus they are not desirable.
The structure shown in FIG. 1 is not optimum for an additional reason. There are two coils for each phase in the stator. The coils, which team or are grouped to form the three phases, are P1 and P4, P2 and P5, and P3 and P6. The coils centers for these grouped coils are separated by 180.degree. mechanical thus 7.times.180.degree.(e). However, for mechanical and electrical alignment of a rotor and stator tooth set in one of the teamed coil phases, there cannot be complete rotor tooth alignment with a stator tooth within the other grouped coil. This forced but undesirable misalignment is seen in FIG. 1, where rotor tooth R1 and stator tooth S1 are completely aligned; but stator tooth S8 on which grouped coil P4 is wound, is slightly misaligned with rotor tooth R5. Since stator teeth S8 and S1 are separated by 7.times.7m=49m and rotor teeth R1 and R5 are separated by 4.times.12m=48m, the misalignment is m=30.degree.(e). The voltages induced in individual coils in the pairs P1-P4, P2-P5 and P3-P6 are similarly out of phase (or antiphase) by 30.degree.(e).
Connection of grouped phase coils P1-P4, P2-P5, and P3-P6 in series will still allow completely electrically balanced operation of the machine shown in FIG. 1 but the materials of construction are not utilized in an optimum manner because of the two sources of electrical phase displacement discussed above.
The machine construction shown in FIG. 1 can be extended to 3n phase machines, where n is an integer, n=2,3,4 . . . , where the total number of stator teeth will be 12n and the total number of rotor teeth will be 7n. If uniform separation of the stator and rotor teeth is again imposed, then the 30.degree.(e) phase displacements shown above for a three-phase machine will still apply, except when n is an even number, in which case complete phase alignment between grouped phase coils is naturally achieved.
Given that the simple, rugged, brushless structure of the AC synchronous machine shown in FIG. 1 is desirable, it would be advantageous to modify its internal geometry to achieve better utilization of its materials of construction and therefore a more cost effective, efficient and/or more power dense machine.